I. Field of the Invention
The present invention relates to dual-band antennas, and in particular to a dual-band antenna system for use within a portable communications device.
II. Description of the Related Art
"Dual-band" portable communication devices are assuming ever increasing importance in the field of wireless communications. For example, dual-band portable phones have been developed for operation over the cellular band (824 to 894 MHz) and the proposed Personal Communication Network (PCN) band (1.8 GHz to 1.99 GHz). A seemingly simple method for effecting transmission and reception of energy over each band would involve using separate antennas, each specifically designed for operation over a given range of frequencies. Unfortunately, the use of separate antennas would increase the cost, size and complexity of the portable phone, particularly if additional space must be allocated for retraction of each antenna.
For portable devices configured with only a single antenna, a straightforward approach of achieving dual-band operation relies on connection of the separate transmit/receive circuitry for each band directly to the antenna. Conventional chains of transmit/receive circuitry typically include, for example, a communications transceiver serially connected to a duplexer. A matching network is also provided for matching the impedance of the antenna to the serially connected duplexer and transceiver. However, in a dual-band device employing only a single antenna, the matching network of a given chain would be required to effect an impedance match between its chain and the combined impedance of the antenna with the other chain. In general, this would entail a complex, difficult design process, since both the impedance of the antenna and of the other chain would vary over frequency.
It is also conceivable that an electronic or electromechanical switch could be used to selectively connect the communications network of each band to the single antenna of the device. Unfortunately, a host of difficulties could be expected to accompany the introduction of such a switch into the dual-band device. For example, the presence of a switch within the antenna signal path would tend to increase signal loss, noise figure, and signal distortion. Moreover, additional electronics would be required to provide a control signal used to command the switch to select a desired communications band. Perhaps more significantly, the utilization of a switch could increase the susceptibility of the device to single-point failure.